In electrophotography, a latent image is created on the surface of an imaging member such as a photoconducting material by first uniformly charging the surface and then selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between those areas on the surface which are exposed to light and those areas on the surface which are not exposed to light. The latent electrostatic image is developed into a visible image by electrostatic toners. The toners are selectively attracted to either the exposed or unexposed portions of the photoconductor surface, depending on the relative electrostatic charges on the photoconductor surface, the development electrode and the toner.
Typically, a dual layer electrophotographic photoconductor comprises a substrate such as a metal ground plane on which a charge generation layer (CGL) and a charge transport layer (CTL) are coated. The charge transport layer contains a charge transport material or an electron transport material. For simplicity, the following discussions herein are directed toward the use of a charge transport layer which comprises a hole transport material as the charge transport compound. One skilled in the art will appreciate that if the charge transport layer contains an electron transport material rather than a hole transport material, the charge placed on a photoconductor surface will be opposite that described herein.
Generally, when the charge transport layer containing a hole transport material is formed on the charge generation layer, a negative charge is typically placed on the photoconductor surface. Conversely, when the charge generation layer is formed on the charge transport layer, a positive charge is typically placed on the photoconductor surface. Conventionally, the charge generation layer comprises a polymeric binder containing a charge generation compound or molecule while the charge transport layer comprises a polymeric binder containing the charge transport compound or molecule. The charge generation compounds within the CGL are sensitive to image-forming radiation and photogenerate electron-hole pairs within the CGL as a result of absorbing such radiation. Examples of charge generation compounds include metal and nonmetal phthalocyanine compounds. The CTL is usually non-absorbent of the image-forming radiation and the charge transport compound serves to transport holes to the surface of a negatively charged photoconductor. Photoconductors of this type are disclosed in the Adley et al., U.S. Pat. No. 5,130,215 and the Balthis et al., U.S. Pat. No. 5,545,499.
In Martin et al., U.S. Pat. No. 5,350,844, there is disclosed a process for the preparation of a `more perfect` crystalline form of the type I polymorph of titanyl phthalocyanine for use as a charge generation compound. The `more perfect` form of type I polymorph of titanyl phthalocyanine is formed by a method which comprises dissolving a precursor type I polymorph of titanyl phthalocyanine in a solution of trihaloacetic acid and alkylene chloride. The resulting solution is then mixed with solvent, enabling precipitation of a type X polymorph of titanyl phthalocyanine. The type X polymorph of titanyl phthalocyanine is separated from the solution and washed. A slurry is formed with the type X titanyl phthalocyanine and an organic solvent which enables conversion of the type X polymorph to a type IV polymorph of titanyl phthalocyanine. The type IV polymorph is then subjected to either (1) organic solvent treatment, or (2) milling to reach the `more perfect` crystalline form of type I polymorph of titanyl phthalocyanine. The milling step comprises a ball mill with stainless steel balls and dichloromethane. The `more perfect` crystalline form of the type I polymorph of titanyl phthalocyanine is disclosed as exhibiting increased photosensitivity when compared to the precursor type I titanyl phthalocyanine.
In Cosgrove et al., U.S. Pat. No. 5,686,213, there is disclosed a method for forming electrophotographic photoconductors which include the steps of selecting a desired sensitivity range and a desired light intensity for a photoconductor and forming the photoconductor having the desired sensitivity range and desired light intensity. Cosgrove et al disclose that sensitivity of perylene pigments increases with milling. Cosgrove et al further disclose that metal phthalocyanines may also be tuned in this manner. Specifically, Cosgrove et al found that the electrical response characteristics of the photoconductor are directly related to the milling time when other factors (such as photoconductor construction) are held constant. That is, Cosgrove et al disclose that as milling time increases, the sensitivity increases. Thus, by selecting an appropriate milling time based on the desired sensitivity, a photoconductor with a specifically desired photo-induced discharge curve may be provided.
The laser printer industry requires a tremendous range of photosensitivities which are dictated by performance constraints of a printer. Specifically, in certain applications, laser printers with decreased photosensitivities are desired. A method of decreasing the photosensitivity of the charge generation layer and photoconductors including the same are therefore desired.